feat: add blog with archives, pagination, search, sharing and RSS

- Content collection (one folder per post) with zod schema
- /blog, category and tag archives with /sayfa/<n>/ pagination
- Post page: breadcrumbs, date badges, share bar, TOC, DIY notice,
  code copy, license line, prev/next, BlogPosting + BreadcrumbList JSON-LD
- Sidebar: Pagefind search, categories, featured fallback, tag cloud
- Sätteri HAST plugins for figure/figcaption and external links
- Dual-theme Shiki, themed prose, RSS feed, latest posts on home
- Three sample posts marked as examples

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/content.config.ts

@@ -0,0 +1,33 @@
+import { defineCollection } from 'astro:content';
+import { glob } from 'astro/loaders';
+import { z } from 'astro/zod';
+
+// One folder per post: src/content/blog/<slug>/index.md(x) + its images.
+const blog = defineCollection({
+  loader: glob({
+    pattern: '**/index.{md,mdx}',
+    base: './src/content/blog',
+    generateId: ({ entry }) => entry.replace(/\/index\.mdx?$/, ''),
+  }),
+  schema: ({ image }) =>
+    z
+      .object({
+        title: z.string().max(70),
+        description: z.string().min(50).max(160),
+        pubDate: z.coerce.date(),
+        updatedDate: z.coerce.date().optional(),
+        category: z.string(), // must exist in src/data/categories.json (checked in lib/posts.ts)
+        tags: z.array(z.string()).default([]),
+        cover: image().optional(),
+        coverAlt: z.string().optional(),
+        featured: z.boolean().default(false),
+        diy: z.boolean().default(false),
+        draft: z.boolean().default(false),
+      })
+      .refine((d) => !d.cover || !!d.coverAlt, {
+        message: 'coverAlt is required when cover is set',
+        path: ['coverAlt'],
+      }),
+});
+
+export const collections = { blog };